Alcohol Use Disorders (AUDs) are highly prevalent and a costly public health problem. Given the compelling evidence for the benefits of increased physical activity for physical health and psychological functioning as well as promising findings regarding alcohol relapse prevention, efforts to help patients with AUDs adopt and sustain increased levels of physical activity (PA) are likely to result in significant public health impact Yet, not only are patients with AUDs engaging in low levels of physical activity, relatively little attention has been given to the development of multilevel physical activity interventions in this unhealthy, at-risk population. Building on the advantages for scalability and dissemination afforded by the use of technology, we propose to develop a smartphone-based physical activity application (app), which we call Fit&Sober, tailored for patients with AUDs. Most existing PA apps suffer from a lack of theory-based concepts known to influence behavior change. However, PA apps that incorporate key behavioral principles relevant to physical activity promotion have demonstrated promising results of increases in physical activity, though none have been examined or tailored for individuals with AUDs. Grounded in Self-Determination Theory and Social Cognitive Theory, we will develop the Fit&Sober app for patients with AUDs by designing features that will help patients develop intrinsic, value-driven goals (e.g., exercisig for benefits related to sobriety) with a particular focus on increasing the saliency of immediate rewards associated with PA (e.g., decreased alcohol urges/craving and improved affect). Early recovery from AUDs, when negative affect is a risk for relapse and urges to drink are common, may be an opportune time to help patients develop internalized, self-determined motivation through experientially making connections between exercise and acute improvements in affect and cravings. The design of this study takes place in 2 phases -- the R21 phase will be focused on the development of the Fit&Sober app, while the R33 phase will be devoted to evaluating the preliminary efficacy of the app in a small RCT. The R21 phase (Years 1 & 2) will consist of 3 stages: 1) development of the Fit&Sober prototype; 2) series of usability studies with patients with AUDs and focus group with addiction treatment counselors; and 3) a 12-week open pilot trial (n=20) to test the feasibility and acceptability of the Fit&Sober app. The R33 Phase (Years 3-5) will involve randomizing 160 patients currently receiving treatment for alcohol addiction at a partial hospitalization program to either: 1) one brief advice (BA) session to increase PA plus the Fit&Sober app OR 2) one BA session. Follow-up assessments will be conducted at 3, 6, & 12-months to determine both the short- and long-term adherence to physical activity and app usage. We expect that this project will lead to the development of a novel, PA app tailored to patients with AUDs, providing them with a valuable adjunct to traditional treatment that can be delivered in any setting and at any time, thereby improving their overall health and well-being.